How do I make my application follow the system theme?

Question

I'm following the standard tutorial for gtk4. A simple hello world application. I compile the program with the given compiler options and run the program. For some reason, the application looks like a gtk3 light-themed application, instead of a gtk4 application, even though I did specify to use the gtk4 library. How do I make my application use the system theme? I want it to also follow light/dark theme.

This is the compiler command that I used:

gcc -fdiagnostics-color=always `pkg-config --cflags gtk4` -g /home/dexterdy/Documents/repos/gtkStart/src/main.c -o ../output/gtkStart `pkg-config --libs gtk4`

#include <gtk/gtk.h>

static void print_hello(GtkWidget *widget, gpointer data)
{
    g_print("Hello World\n");
}

static void activate(GtkApplication *app, gpointer user_data)
{
    GtkWidget *window;
    GtkWidget *button;
    GtkWidget *box;

    window = gtk_application_window_new(app);
    gtk_window_set_title(GTK_WINDOW(window), "Window");
    gtk_window_set_default_size(GTK_WINDOW(window), 200, 200);

    box = gtk_box_new(GTK_ORIENTATION_VERTICAL, 0);
    gtk_widget_set_halign(box, GTK_ALIGN_CENTER);
    gtk_widget_set_valign(box, GTK_ALIGN_CENTER);

    gtk_window_set_child(GTK_WINDOW(window), box);

    button = gtk_button_new_with_label("Hello World");

    g_signal_connect(button, "clicked", G_CALLBACK(print_hello), NULL);
    g_signal_connect_swapped(button, "clicked", G_CALLBACK(gtk_window_destroy), window);

    gtk_box_append(GTK_BOX(box), button);

    gtk_widget_show(window);
}

int main(int argc, char **argv)
{
    GtkApplication *app;
    int status;

    app = gtk_application_new("org.gtk.example", G_APPLICATION_FLAGS_NONE);
    g_signal_connect(app, "activate", G_CALLBACK(activate), NULL);
    status = g_application_run(G_APPLICATION(app), argc, argv);
    g_object_unref(app);

    return status;
}

Answer 1

What do you mean by GTK4 theme? Is that theme what the new gnome settings use? That's not actually a theme, but using libadwaita (uses another theme). If you create a AdwApplication and AdwWindow instead of a GtkApplication and GtkWindow you should see the same theme. Don't really know how to use the same theme on a pure GTK4 Application.